1. Field of the Invention
This invention relates generally to the field of exploration and production of hydrocarbons. More specifically, the invention relates to a method of determining reservoir pressure in hydrocarbon reservoirs.
2. Background of the Invention
Increasingly, hydrocarbon resource is being developed in low permeability reservoirs. For example, more than 50%) of the currently identified gas resources in the United States is found within tight gas and shale gas reservoirs (ref. Table 9.2 of the US EIA report titled “Assumptions to Annual Energy Outlook 2011” http://38.96.246.204/forecasts/aeo/assumptions/pdf/0554(2011).pdf). Estimation of reservoir pressure in tight gas, shale gas and tight oil reservoirs is important for variety of reasons including estimates for ultimate recovery, production forecasting and optimization of depletion planning. Completion designs, modeling studies and future field development decisions are all improved with better knowledge of current and historical reservoir pressures. However, reservoir pressure is difficult to determine in low permeability reservoirs using current well testing methods due to the long shut-in times required for the well test to obtain a reliable estimate of reservoir pressure. Injection testing during the completion phase performed prior to hydraulic fracturing in many cases has proven effective in determining reservoir pressures in low permeability reservoirs. These types of tests become less economically attractive to run if reservoir conditions are such that the fluid level within the wellbore drops below surface resulting in the well going on vacuum before sufficient data has had time to be collected. Running downhole pressure gauges is often not economically practical. In addition, as the permeability decreases, the monitoring time required increases, introducing a significant delay in the well completion. Obtaining reservoir pressure in a more economical manner has value in a number of ways. In marginally economic developments, it may present the only way to directly measure reservoir pressure. Mini falloff tests (MFoT) are the current accepted industry practice to obtain estimates of reservoir pressure in low permeability reservoirs. While MFoTs are generally conducted prior to hydraulic fracture treatments the time to obtain analyzable data could still require several days thus limiting the application of this method on a wide scale basis. The main reasons for this are wellbore storage and the need to obtain pseudo radial flow. Having a method where wellbore storage and obtaining radial flow were not limiting factors would greatly enhance the ability to obtain estimates of reservoir pressure. Should an MFoT be performed, having a prior independent estimate of reservoir pressure would allow for both a confirmation of reservoir pressure and improvements in permeability height estimates. If reservoir pressure and permeability are known then during production, rate transient analysis (RTA) can be more accurately applied to estimate fracture length and conductivity, thus allowing for improved understanding of completion efficiencies. Utilizing test results to establish relationships between reservoir pressure (pore pressure) and fracture and/or closure pressures within specific reservoir intervals, could potentially allow for using historically recorded fracture and/or closure pressures to provide estimates of historical field wide depletion. Documented results utilizing Eaton's correlation shows that comparing measured pore pressure to fracture and/or closure pressure could result in establishing trend lines for specific reservoirs. Applying previously documented fracture and/or closure pressures to these trend line relationships would yield estimated reservoir pressure at the time of each of the prior fracture simulations, thus providing estimates of historical fieldwide depletion. Establishing and utilizing these trend lines then could greatly leverage the test results far beyond the limited amount of actual test data actually measured. To have confidence in these trend lines a statistically meaningful set of measures reservoirs pressures must be taken.
Consequently, there is a need for an improved low-cost and accurate method of determining reservoir pressure in low permeability reservoir systems.